In most hydrocarbon treatment processes it is often necessary at some point in the process to contact and then separate two or more liquids from each other based on density differences. One approach is to use Merichem Company's FIBER FILM® contactor technology in combination with a horizontal settling vessel. These fiber-film type separators/contactors are described in U.S. Pat. Nos. 3,758,404; 3,977,829 and 3,992,156, all of which are incorporated herein by reference. The basic design of a fiber-film type contactor/separator comprises a plurality of vertical hanging fibers contained either totally or partially within a vertical shroud that is configured to maintain the liquids within the inside of the shroud and in contact with the hanging fibers as the liquids flow downward parallel to the axis of the fibers and shroud. Once the liquids exit the shroud they enter a separation section, typically a horizontal settler, where gravity separates the phases, with the lower density liquids forming an upper layer and the higher density liquids forming a lower layer. Each layer can then be selectively removed from the separation section of the apparatus.
Separation efficiency is a function of the interfacial tension of the liquids to be separated; however, residence time and surface area of the separation device are also important variables. Low interfacial tension (IFT) of the liquids in the admixture causes formation of emulsions and high dispersions, which manifest in the absence of a clear well defined phase boundary interface. The term “dispersion” is understood to mean a two-phase liquid mixture that is in the process of separation over the time frame of seconds to minutes in the vessel, as opposed to individual droplets of one phase in another phase. A poor separation could simply be tolerated, however, it would then become necessary to force the effluent through a separate mechanical separation device, such as packed bed, centrifuge, centrifugal liquid separator, hydro cyclone or the like. These solutions have disadvantages of high capital cost and additional energy input. In addition to mechanical attempts to cure the problem, sometimes the addition of chemicals is used in an attempt to encourage the liquid phases to separate (in a similar way as antifoams are used to resolve gas-liquid mixtures). This incurs additional expense and is typically not very effective because with the low interfacial tension of the system the addition of surface-active chemicals may make the problem worse.
This dispersion problem occurs in hydrocarbon treatment processes, for example, the desulfurization of kerosene using an aqueous catalyst solution via an oxidation reaction. In such processes two liquid phases are created that can be very difficult to separate downstream because of the low interfacial tension of the two liquid phases. When using a fiber-film type contactor/separator the two-phase mixture does not travel exclusively down the tail of the fibers, but tends to spread out as a loose dispersed liquid mixture. The mixture stays together and has the visual appearance of foam, forming a so-called “dispersion band”. If the flow of either liquid is stopped, the dispersion band may ultimately collapse over time. Even when the liquid flows are not stopped, the dispersion can collapse on its own, at a rate depending on the vessel system and properties of the liquids, especially their interfacial tension. However, if the production rate of this dispersion is higher than the collapse rate, the dispersion band will grow in volume, piling up in the vessel. This will then lead to a high carryover of the heavy liquid into the light overhead product. Although the art has used coalescers for liquid/liquid service, they are usually designed to handle only small amounts of a dispersed liquid. One example is a candle filter made of a suitably hydrophobic or hydrophilic medium, or both, that forces the two phase mixture to adhere to one or the other surface and form larger droplets, which are then easy to separate by gravity. Such devices unfortunately suffer from the disadvantage that it adds extra pressure drop and pumping costs to the overall process. Additionally, larger amounts or slugs of such dispersions can easily overcome these known coalescers.
Another problem of this accumulation of dispersed liquid mixture is the lack of a clear interface between the liquid phases in the lower section of the separation vessel. This may lead to difficulty in detecting the interface by traditional instrumentation such as capacitance probes, guided-wave radar instruments, level floatation switches and the like. Lack of robust level detection makes controlling the liquid interface level very difficult. Moreover, this lack of a defined interface could result in the true pure liquid level dropping to the point at which pumps cavitate, or where the light liquid is drawn into the pump suction, leading to severe operating problems. The problem of forming a two-phase dispersion is particularly severe when the interfacial tension (IFT) is less than 10 dynes/cm as measured by standard methods using an interfacial tensiometer.
Up until the present invention, the solution to resolving such a two-phase dispersion was to use a very large horizontal vessel to disengage or collapse the dispersion over a relatively long residence time. The large horizontal surface area in the vessel allows the dispersion to form a relatively thin dispersion band or “rag layer,” which has sufficient residence time to collapse. Such large horizontal vessels, for example as depicted in Frank, T. C. et. al. “Liquid-liquid dispersion fundamentals”, Perry's Chemical Engineers' Handbook, 8th ed. 2008 chapt. 15 p. 98, have large capital costs and need a large foot print that occupies valuable real estate. The present invention solves these problems by using an enhanced coalescing zone that contains the hanging fibers and allows a portion of liquids to flow out of a disengagement device where it contacts a coalescing surface in a non-parallel path relative to the vertical axis of the hanging fibers.